St. James's Palace says the couple found out about the pregnancy "recently" and that members of both families "are delighted with the news."

Acute Morning Sickness

The duchess is being treated for hyperemesis gravidarum, also known as HG, at the King Edward VII Hospital in London. It causes constant nausea and vomiting, often needing admission to a hospital.

"As the pregnancy is in its very early stages, Her Royal Highness is expected to stay in the hospital for several days and will require a period of rest thereafter," St. James's Palace said in a statement.

Not being able to keep food and fluids down raises the risk of dehydration and kidney problems, which is why urgent medical treatment is needed.

"It can be really horrible," says professsor Tim Draycott from the University of Bristol. He is a consultant obstetrician and spokesman for the Royal College of Obstetricians and Gynaecologists. "About 30% of women get some sort of sickness in pregnancy. About one in 200 to one in 100 women get the very severe version, the hyperemesis that Kate has at the moment.”

"It's a very uncomfortable condition. You feel very unwell."

He says Catherine will receive fluids through an IV to treat dehydration before being allowed to go home. However, if the extreme sickness returns, Catherine may need to return to the hospital, he says.

"It's absolutely miserable," says Rosie Dodds, senior policy adviser at NCT, the UK’s largest charity for parents. "The continual retching on an empty stomach is something people find very difficult to cope with."

She says smells can often be a trigger, such as smoke, alcohol, or gas. "Sometimes just the thought of food makes them feel more sick."

Draycott says the condition usually eases after 12-13 weeks, but can last past 20 weeks in rare cases.

It can lead to weight loss in pregnancy, which could result in a baby having a low birth weight. Still, Draycott says it’s unlikely to cause harm to the growing baby. "There's a very tiny chance of it affecting the baby's growth."

The couple's first child would be third in line to the throne, after Prince Charles and William.